There are vast deposits of oil shale throughout the world with some of the richest deposits being in the western United States in Colorado, Utah and Wyoming. These reserves are regarded as one of the largest untapped energy reserves available. The oil shale is in the form of solid rock with a solid carbonaceous material known as kerogen intimately distributed therethrough. The kerogen can be decomposed to a synthetic crude petroleum by subjecting it to elevated temperatures, in the order of 900.degree. F. This causes the kerogen to decompose to a hydrocarbon liquid, small amounts of hydrocarbon gas and some residual carbon that remains in the spent shale. The heat for retorting the shale oil can be obtained by burning some of the carbonaceous material in the shale with air or other oxidizing gas.
Preferably the oil shale is retorted in situ in a bed of oil shale particles filling a cavity blasted into the undisturbed oil shale. In such an in situ retort the rubble pile of shale particles is ignited preferably at the top and air is passed downwardly through the bed to sustain combustion and retort the oil. Liquid oil flows to the bottom of the retort and is recovered.
Such retorts can be formed, for example, by excavating a portion of rock in a volume that ultimately will become an underground retort. The balance of the rock in the volume to become a retort is then explosively expanded to form a rubble pile or bed of oil shale particles substantially completely filling the retort volume. The original excavated volume is thus distributed through the expanded oil shale particles as the void volume therebetween.
Oil is then extracted from the expanded rubble pile in the underground retort by igniting the top of the bed of oil shale particles and passing an oxygen bearing gas, such as air, downwardly through the retort. Once raised to a sufficient temperature the oil shale will support combustion, initially at the top of the retort by burning some of the oil in the shale. Thereafter, as the oil is extracted there is residual carbon left in the shale, and, when at a sufficient temperature, this too will react with oxygen to burn and supply heat for retorting. This burning of residual carbon in the shale depletes oxygen from the air being passed down through the retort and the substantially inert gas then carries heat to a retorting zone below the reaction zone for decomposing the kerogen and extracting oil. Gases from the bottom of the retort are collected and often contain sufficient hydrogen, carbon monoxide and/or hydrocarbons to be combustible. Oil is also collected at the bottom of the retort and transported for conventional refining.
After retorting of the shale oil is completed, the retort contains a large volume of hot spent shale. This heated spent shale contains a substantial amount of unburned residual carbon. Some combustion does occur in the heated spent shale during retorting by reaction between oxygen and residual carbon. In a typical retorting operation only about 46% of the residual carbon resulting from retorting was consumed during the retorting operation. The other 54% of the residual carbon remained in the spent shale at the end of normal retorting operations. Appreciable quantities of recoverable energy in the form of sensible heat or unburned carbon may remain in the spent shale.
When the oil shale is expanded in the underground retort the particles ordinarily fill the entire volume so that there is no significant void space above the rubble pile. Air for combustion can be brought to the top of the bed of particles by means of holes bored through overlying intact rock. Appreciable difficulty may be encountered, however, in igniting the top of the rubble pile to support combustion. Ignition requires a substantial amount of heat delivered over a sufficient time to raise a reasonable volume of oil shale above its ignition temperature. Some difficulty is encountered in heating a substantial volume of oil shale in the retort and assuring that ignition has been obtained.